Government standards associated with combustion engine emissions have increased the burden on manufacturers to reduce the amount of nitrogen oxides (NOx) and particulates that may be exhausted from their developed engines. Along with this burden is the manufacturer's commitment to their customers to produce fuel efficient engines. However, the sometimes inverse relationship between fuel economy and reduced emissions tends to make the task of reducing NOx while meeting customer needs a daunting one.
NOx emission levels may be affected by engine combustion temperatures and air to fuel-vapor ratio, among other things. When the temperature inside combustion chambers exceeds 1300 degrees C., nitrogen may combine with oxygen to form oxides of nitrogen, or NOx. Because lean mixtures in a power source typically lead to higher combustion temperatures, lean burn engines may produce more NOx than other richer burning power sources. Some engines rely on methods such as exhaust gas recirculation, for example, to lower combustion chamber temperatures and reduce NOx formation. These methods may be insufficient to meet standards promulgated by government agencies limiting NOx emissions.
Selective catalytic reduction (SCR) provides a method for removing NOx emissions from fossil fuel powered systems for engines, factories, and power plants. During typical SCR, a catalyst may facilitate a reaction between exhaust gas NOx and a reductant, for example, ethanol, to produce nitrogen gas and byproduct substances such as water, nitrogen, and acetaldehyde, thereby removing NOx from the exhaust gas.
Reductants used in an SCR system have previously been injected into the exhaust-gas stream upstream of a catalyst and mixed with the exhaust gas to facilitate a reaction in the presence of the catalyst. Thorough mixing of the reductant in the exhaust-gas stream may improve the reaction between the reductant and NOx, thereby further reducing NOx emissions and limiting the release of highly-reactive species into the atmosphere. The performance of a lean-NOx catalyst to reduce NOx may depend upon many other factors, such as catalyst formulation, the size of the catalyst, exhaust gas temperature, the reductant compound, and reductant dosing rate. The result has been to somewhat reduce atmospheric output of NOx, but reduction has fallen short of governmental requirements.
Anhydrous fuel-grade ethanol has been used with some success as a reductant in SCR systems through injection into an exhaust-gas stream upstream of an SCR system catalyst. In such a system, NOx in the exhaust-gas stream may react with the injected ethanol in the presence of the catalyst which may result in formation of acetaldehyde, nitrogen, water, and other byproducts. However, anhydrous fuel-grade ethanol is known to be highly reactive and difficult to store and maintain in its pure state. Further, injection of the ethanol into the exhaust stream is a waste of energy otherwise available within the ethanol.
Fuel-grade ethanol has also been emulsified within diesel fuel for combustion in quantities up to approximately 15% ethanol by volume as a means for increasing consumption of renewable type fuels and reducing some pollutant emissions. This emulsification has been accomplished using proprietary emulsifying agents to maintain some stability in the emulsion and reduce reactivity. However, emulsified ethanol is still highly corrosive and lacks the lubricating qualities of petroleum based fuels. This may result in long-term damage to injection pumps and fuel injectors designed to receive petroleum based fuels exclusively. Further, emulsions of ethanol within petroleum fuels greater than 15% ethanol by volume, create unstable, reactive emulsions and are, therefore, impractical for storage or use in an engine. Further, because the anhydrous fuel-grade ethanol is emulsified in low concentrations and designed for combustion, a majority of the emulsified ethanol is combusted in the combustion chamber, resulting in little if any remaining ethanol to be used as a reductant in the exhaust-gas stream.
One system for using fuel-grade anhydrous ethanol as a reductant in a lean-NOx SCR system is disclosed in the publication Selective Catalytic Reduction of Diesel Engine NOx Emissions Using Ethanol as a Reductant, U.S. Department of Energy 9th Diesel Emissions Reduction Conference (Aug. 24-28, 2003) by Kass et al. (hereinafter “the Kass publication”). The system of the Kass publication includes an injector for spraying ethanol, which is either extracted from e-diesel or stored separately in a fuel-grade anhydrous form, directly into a bent region of the exhaust pipe to facilitate mixing of the ethanol and exhaust-gas stream. The system further includes a system for extracting a portion of fuel-grade ethanol from e-diesel which may be stored in a fuel storage tank. An ethanol injector is placed upstream of an alumina-supported silver lean-NOx catalyst such that conversion of NOx is facilitated as the mixture contacts the lean-NOx catalyst.
While the system of the Kass publication may result in some NOx reduction through ethanol introduced in the exhaust stream, both e-diesel and fuel-grade ethanol can be more difficult to store and manage because of their reactive characteristics. As a result, added cost may be incurred when using e-diesel and/or fuel-grade ethanol as a reductant injected into an exhaust stream.
In addition, injection of ethanol into an exhaust stream, as taught in the Kass publication, may not result in adequate mixing of the ethanol with the exhaust-gas stream, and, consequently, may result in discharge of unreacted fuel-grade ethanol. Moreover, injection of ethanol into the exhaust stream may deprive the engine of valuable energy stored within the ethanol, thereby eliminating any benefit to brake specific fuel consumption.
The present disclosure is directed at overcoming one or more of the problems or disadvantages in the prior art exhaust gas mixing systems.